A HBT technique has been examined primarily for improving the high-speed performance of the bipolar transistor. A HBT forming method examined by the present inventors is, for example, described as follows.
First, on a semiconductor substrate, a silicon nitride film, a polycrystalline silicon film for forming a base electrode, and a silicon oxide film are deposited in order of this subsequently to a lower layer. Then, after forming a first opening-forming photoresist pattern on the silicon oxide film, the silicon oxide film and the polycrystalline silicon film that are exposed therefrom are etched in order of this. Thereby, a first opening portion is formed in the silicon nitride film and the polycrystalline silicon film in such a manner as to expose a portion of an upper surface of the silicon nitride film from a bottom portion. Then, after forming a sidewall-insulation film on a side surface of the first opening portion, a second opening portion greater in a plane size than the first opening portion is formed in such a manner as to communicate with the first opening portion by removing the silicon nitride film through the first opening portion. From this second opening portion, a main surface of the semiconductor substrate and a portion of a bottom surface side of the polycrystalline silicon film are exposed. Then, in the second opening portion, a dissimilar crystalline layer such as silicon-germanium (SiGe) is made to grow selectively by an epitaxial method. This dissimilar crystalline layer is formed by being made to grow from both an exposed surface side of the semiconductor substrate and an exposed surface side of the polycrystalline silicon film. Thereafter, a polycrystalline silicon film for an emitter electrode is embedded in the first opening portion, and impurities in the polycrystalline silicon film are diffused in the dissimilar crystalline layer to form an emitter region. Note that these HBT forming techniques are disclosed in, for example, Japanese Patent Publication No. 2705344 or Fumihiko Sato, et al., “A Super Aligned Selectively Grown SiGe Base (SSSB) Bipolar Transistor Fabricated by Cold-Wall UHV/CVD Technology”, IEEE Trans. ED, Vol. 41, pp. 1373–1378 (1994).
However, it has been found at first by the investigations of the present inventors that the above HBT forming method has the following problems. That is, in the above method, an upper portion of the silicon nitride film on the bottom surface of the first opening portion is slightly etched when the first opening portion is formed. In particular, if the photoresist film is used as an etching mask at the time of forming the first opening portion, the silicon nitride film is easily etched because it is impossible to provide a sufficiently high selective ratio of the polycrystalline silicon film to the silicon nitride film. If the second opening portion is formed after forming the sidewall-insulation film on the side surface of the first opening portion under this condition as described above, a lower portion of the sidewall-insulation film becomes greatly protruded to a side of the second opening portion along a direction orthogonal to the main surface of the semiconductor substrate. If the above-mentioned dissimilar crystalline layer is made to grow under this condition, the growth of the dissimilar crystalline is blocked by the protrusion of the sidewall-insulation film. In particular, since the growth of the dissimilar crystalline is blocked at a portion of a bottom surface side of the polycrystalline silicon film for forming the base electrode, the dissimilar crystalline layer is unsuccessfully connected to the polycrystalline silicon film, whereby there is the problem that base resistance is greatly increased.
An object of the present invention is to provide a technique for being capable of improving reliability of a semiconductor device having a HBT.
The above and other objects and novel features of the present invention will become apparent from the description of this specification and the drawings.